This invention relates to the field of compacting presses for powder materials, and in particular to such presses as are used to compact powder metal into the form of gears, helical gears, and most particularly to opposed double helical, or herringbone, gears.
Powder compacting presses have been known for many years. They typically involve at least three interacting pans: a die, an upper punch and a lower punch. Initially the top punch is separated from the die and powder is introduced into a cavity formed within the die above the lower punch. Subsequent motion of the opposed punches reduces the internal cavity volume to compress the powdered metal to desired density. The resulting green formed pan is removed from the cavity and sintered. For a part having sections of differing thickness additional movable top or bottom punches may be added to promote transfer of powder within the cavity.
The manufacture of gear teeth is more difficult when a helical gear is desired. Unlike a simple spur gear, as the die for a helical gear is closed it must also rotate relative to the punch, and then must achieve relative rotation in the opposite direction to release the compacted pan. Where the helix angle is shallow, and the thickness of the gear is modest, an externally helically threaded punch is, or opposed punches are, brought into a mating, internally helically threaded die under the pressure of longitudinally acting rams. The die and one punch or both punches are carried in bearings and the force of the ram acting against the threads causes the tool elements (i.e. die and punch or both punches) to auto-rotate. Auto-rotating helical tool elements (i.e. die and punch or both punches) are known, as for example in U.S. Pat. No. 3,694,127 to Takahashi et al., and U.S. Pat. No. 5,259,744 to Take.
When the helix angle or the thickness of the gear increases, the frictional resistance in such dies may become large. To overcome this friction it is known to use motors to apply a torque to the tool elements, or to cause rotation of the tool elements at an appropriate speed, given the helix angle, as longitudinal rams force the tool elements together. It is also known that if one wishes to make parts having keyways or eccentric bores or internal splines there must be no relative rotation of the punch or core feature relative to the compacted powder, since such motion would shear off the keyway or bore.
Powder metal gears with offset, phased or undercut upper and lower portions have been produced. In these cases the finished pans can comprise at least two gear profiles formed in opposing dies which separate on a parting plane. In the case of helical gears it would be advantageous to be able to produce a gear having a helical profile to one side of the parting plane of the dies, and a different profile to the other side, whether an opposed helix, a helix of different pitch of the same hand, or out of phase helix, or a spur gear, whether of the same diameter or tooth height or not. A typical application of this kind of technology relates to the production of symmetrical opposed helical gears, most often referred to as herringbone gears.
It is advantageous to make herringbone gears from compacted and sintered powder metal since it is difficult and expensive to machine herringbone gears in the conventional manner. Conventional powder metallurgy may instead require back to back placement and juncture of two opposite-handed helical gears. This limits the sire and delicacy of the metal herringbone gears that can be manufactured, and also their quality. If welded together such gears may not be true. If mechanically fastened such gears may be unnecessarily bulky.
To date the inventor is unaware of any powder metal presses for producing double opposed helical, or herringbone, gears. U.S. Pat. No. 3,694,127 to Takahashi et al. shows, at FIGS. 11 and 12, a powder metal compact and tooling for opposite handed helical threads. This apparatus cannot be used to produce herringbone gears, or even opposite handed gears in which the diameter of the gears is close, since, as noted in U.S. Pat. No. 5,259,744 to Take, the outer lower punch wall becomes too thin. Experience suggests that the minimum die wall thickness required to make a reliable tool is about 2 mm., which with allowance for the dedendum of the larger gear and the addendum of the smaller gear, would limit the parts which can be produced. The Takahashi device also relies on auto-rotation to move the upper punch, die, and lower outer punch all at once. Take can be used to make two helical gears of the same hand, but once again cannot make herringbone gears and is limited to producing helical gears that vary in diameter by at least the height of the teeth to be produced.
Thus there is a need for a device and method for compacting powder to form opposed twin helical gears that avoids thin walled punches. Further, there is a need for a device and method capable of compacting powder not only to form herringbone gears, but also to form opposed handed helical gears of even very small differences in diameter.
More generally, there is a need for a powder metal tool set that may be used to produce two-part helical gears, whether those two pans are of the same diameter or not.
The present invention concerns a multiply-acting powder compacting press and methods for operating that press to produce two pan gears of a variety of types, in particular for producing powder metal symmetrically opposed helical, or herringbone, gears and two pan helical gears whose diameters are substantially the same.
A powder metal multiply-acting press for the purposes of the present invention has a tool set having a core rod, an inner lower, or transfer, punch; an outer lower punch; a lower die; an upper die; and an upper punch. The upper portion (for example Sheet 2 of 2) may comprise an upper outer punch and an upper inner, or pre-lift, punch to aid lateral transfer of powder.
Depending upon the type of gear to be produced the present invention pertains to tool sets in which either two or three elements rotate during the compaction and withdrawal steps of pressing a green powder metal compact.
In a first aspect of the invention there is a tool set for making double helical gear compacts, that tool set comprising a lower punch having a first helical gear profile and a lower die having a mating negative helical profile for helically sliding engagement with the lower punch; an upper, opposed punch having a second helical profile, an upper die having a mating negative helical profile for helically sliding engagement with that upper, opposed punch; that upper punch disposed in opposition to said lower punch; and those lower and upper dies movable to abut at a parting plane.
In a another aspect of the invention the press includes a tool set for making opposed, double helical gear compacts, the tool set comprising a first punch having a Dust helical gear profile; a Dust die having a negative helical profile for mating with the Dust punch; a second, opposed punch having a second, opposite handed, helical profile; a second die having a negative helical profile for mating with the second, opposed punch. The tool set is movable to a filling position for receiving a charge of powder metal; a transfer position; a compaction position; and a withdrawal position; a) in the transfer position the dies are disposed in longitudinally abutting, unrotated relationship; the punches are in a first, retracted, opposed, spaced apart relationship; whereby a cavity for containing the powder metal charge is defined longitudinally by the opposed faces of the punches and peripherally by the dies; b) in a compaction position the punches are in a second, advanced, opposed, spaced apart relationship; the dies remain in abutting relationship; and the dies are moved to a partially rotated position whereby the cavity is reduced in size to compact the powder; c) in a withdrawal position the punches remain in an advanced, opposed, spaced apart relationship; and the dies are disposed in a fully rotated position whereby moving the dies to a fully rotated position causes the dies to separate and expose a compressed workpiece. Furthermore, the tool set may comprise a transfer punch surrounded by the first punch; and in the transfer position such transfer punch being in an advanced position to urge the powder charge to spread throughout the cavity.
The invention may further involve a pitch drive for coordinating rotation of the dies during longitudinal translation of the punches, the pitch drive receiving mechanical input from the motion of at least one of die punches and providing output to at least one of the dies; and that pitch drive may be a cam and roller mechanism, one of a) a cam or b) a roller in rigid structural relationship to one of the punches whether upper or lower; the other of a) the roller or b) the cam in rigid structural relationship with the corresponding upper or lower die, whereby longitudinal translation of that one of the punches relative to that one of the dies compels rolling engagement of the roller and the cam and consequential relative rotation of that die with respect to that punch.
A third aspect of the invention involves a method for using a tool set to make powder metal opposed helical gear compacts, that tool set having a First punch having a first helical gear profile; a first die: having a mating helical profile for mating with the first punch; a second, opposed punch having a second, opposite handed helical profile; a second die having a helical profile for mating with the second, opposed punch; and a transfer punch, the method comprising moving the tool set to a filling position; introducing a charge of powder metal to the tool set; moving the tool set to a transfer position in which the dies are in longitudinal abutting relationship and the punches are in a retracted, spaced apart relationship, a cavity being formed within the dies between the opposed punches; moving the transfer punch relative to the cavity to distribute the charge throughout the cavity; compacting the powder in the cavity to form a workpiece; withdrawing the dies to expose the workpiece; and removing the workpiece. The step of compacting the powder may be achieved by maintaining the first punch in one position. advancing the second punch toward the first punch; translating the dies longitudinally in the same direction as the second punch while simultaneously rotating the dies, the step of moving the transfer punch may be achieved by holding the transfer punch stationary and advancing the other punches and the dies in unison.
In a fourth aspect there is a method for making asymmetric double helical gear compacts in a multiply acting press, that method comprising the steps of a) filling a cavity lower portion with a charge of powder; displacing the upper and lower dies to abut at a parting plane with opposing distal end faces of upper and lower punches proportionately distant from a parting plane; c) displacing a transfer punch to distribute the charge of powder throughout the cavity; d) compacting the charge of powder to form a powder compact by advancing upper, lower, and transfer punches proportionately toward the parting plane while the upper punch rotates relative to the upper die and the lower punch rotates relative to the lower die; e) withdrawing both of i) the upper die along said upper punch, and ii) the lower die along the lower punch, during relative rotation of the dies relative to the punches and the powder compact, to a first withdrawal position in which one of the dies clears the powder compact; withdrawing the other of the dies along its mating punch, to a second withdrawal position in which the other die clears the powder compact; and g) ejecting that powder compact.
In a fifth aspect of the invention one finds gears that can be produced with the tool sets described. Those gears include double helical, sintered powder metal gears that differ in pitch diameter only by a small amount, such as twice the sum of the dedendum of the larger gear, the addendum of the smaller gear and 2 millimeters. That aspect of the invention includes gears of substantially equal: diameter, and, in particular. herringbone gears.